Ionic conductivity is usually associated with the flow of ions through a liquid solution of salts. In the vast majority of practical uses of ionic conductors, i.e., as electrolytes for dry cell batteries, the liquid solution is immobilized in the form of a paste or gelled matrix or is absorbed in a separator to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the use of a large volume of immobilizing material has hindered the aims of miniaturization.
In addition, improved microelectronic circuit designs have generally decreased the current requirements for electronic devices. This in turn has enhanced the applicability of solid electrolyte power sources which usually can deliver currents only in the microampere range. These solid electrolyte systems have the inherent advantages of being free of electrolyte leakage and internal gassing problems due to the absence of a liquid phase. In addition, they also have a much longer shelf life than the conventional liquid electrolyte power sources.
In attempting to avoid the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds hoping to find compounds which are solid at room temperature and have specific conductances approaching those exhibited by the commonly used liquid systems.
However, numerous technological problems are encountered in the construction of totally solid state electrochemical cells, particularly in the establishment of anode/electrolyte interfaces. The low electrode impedances required for good performance of electrical cells can be achieved by bringing the solid surface of the electrode into intimate contact with the solid electrolyte. Traditionally, this contact has been achieved by stacking the cell components in a mold between dies and applying uniaxial force to densify and compress the components. Great uniaxial force is required to deform solid materials. Typically, the construction of high energy density solid state electrochemical cells requires the use of malleable alkali metal or alkaline earth metal anodes. However, the forces necessary to establish satisfactory interfaces, especially anode/solid electrolyte interfaces, exceeds by a large measure the force needed to cause such malleable metal anodes to flow or extrude radially across the cell stack. This flow of metal can often result in the anode contacting the cathode, thereby short circuiting the cell. An additional and equally serious difficulty is the lamination of the solid electrolyte layer when subjected to the high uniaxial forces needed to establish interfaces and densify the solid electrolyte.
Other approaches have been employed in the prior art to establish satisfactory anode/electrolyte interfaces in solid state electrochemical cells. For example, vapor deposition of the anode metal onto the solid electrolyte can provide an acceptable anode/electrolyte interface. Vapor deposition of alkali metals and alkaline earth metals is, however, a hazardous procedure. Another approach involves blending finely divided anode metal with solid electrolyte to produce a high surface area anode and impart a high surface area anode/electrolyte contact. This also often produces an acceptable contact but the necessity of handling finely divided alkali metals or alkaline earth metals poses a serious fire hazard. Additionally, the dilution of the anode metal with solid electrolyte reduces the energy densities of solid state cells.
The problem of insuring good contact between the components of solid state electrochemical cells exists even in those cell systems wherein the electrolyte is formed in situ by the reaction of the cathode and the anode. Thus, U.S. Pat. No. 3,937,635 discloses that, in lithium-iodine cells, air gaps which remain between the cathode and anode after assembly can, through lithium nitride formation, form an internal electrical short circuit in the cell and if the cathode material does not completely contact the anode an abnormally high impedance can build up at the small remaining interface.
Failure to establish satisfactory interfaces may manifest itself in high cell impedance and poor discharge performance. Accordingly, it is an object of the present invention to provide a method for producing solid state electrochemical cells having improved electrode/electrolyte interfaces.
It is another object of this invention to provide a method for the production of solid state electrochemical cells wherein there is improved contact between the solid electrolyte and a malleable metal anode.
It is yet another object of this invention to provide a method for producing a solid state electrochemical cell utilizing high pressures wherein the dangers of extrusion or radial flow of the anode metal from its desired location in the cell stack and of lamination of the solid electrolyte are reduced.
The foregoing and additional objects will become more fully apparent from the following description of the invention.